Imagine that you want to get a job and are at a job interview. If you accidentally slip and say a bad word, you won't be hired. If you don't get into this bad habit now, you won't have to worry about that.
I can't hire you now that I know you say bad words regularly!
Also, it doesn't actually make you sound cool. It can make you sound very uncool, or even unintelligent. Saying bad words also annoys lots of people.
Saying them can also get you into trouble.
What did you just say!?
A bad word. I'm sorry.
For these reasons, and many others, don't say bad words!